we are currently converting our backups from perodic full backups to Forever Forward Incremental Backups with 30 restore points and are asking ourselves for the best setting in the Maintenance tab, 'remove deleted items data after', which has a default setting of 14 days. In my naive understanding this value of 14 days contradicts the setting of 30 restore points, because then there are restore points from which previously deleted data cannot be restored, isn't it? Does it make sense to set the same value here as for the number of restore points? What is a best practice recommendation in this question?

When the threshold is reached the data blocks related to the excluded VM(s) will be removed from the entire backup chain.
Please read this article about Retention Policy for Deleted items in the User Guide. Thanks!

okay, then I totally misunderstood this feature. I thought it was about removing deleted data from a backup after n days, a quasi-negative synchronization. Meanwhile it is about whole VMs, which is of course a completely different scenario.

The deletion from the backup of data already deleted on the working server would have to take over by the retention policy. If a restore point has expired that contains deleted data for a possible restore, then the data from the full backup will also be removed when this restore point is deleted. That was actually the motivation that made me think through this feature with the 'Remove deleted items data after'.